1. Technical Field
The present invention concerns the field of producing components for trimming or interior lining for automotive vehicles, more specifically components of this type comprising or consisting of a sheet or plate of thermoplastic material, shaped in an adapted manner and which may or may not have relief patterns, shaped into the solid material.
The subject of the invention is, more specifically, a molding device for the manufacture, by negative vacuum shaping, of parts from sheets or plates, and a manufacturing process that uses this device and a trimming component obtained by this process.
2. Related Art
Different processes used for the manufacture of parts by forming under negative pressure (better known under the English-language designation “Negative Vacuum Forming”) are already known in the state of the art (see for example: FR 2 900 862, US 2013/0221698).
In general, in this type of process, a sheet or a plate made of plastic material is pre-heated, then placed between two parts of a mold (or half-molds) which pre-form it during the closing process and, after the mold has been fully closed, a relative vacuum is applied to the part of the mold that comes into contact with the visible surface of the part, resulting in the final forming of the part. Eventually, while pressure is being applied as a result of the surface suction, relief patterns or embossing are made on the visible surface (negative embossing present on the part of the mold that is subjected to suction).
This method of vacuum forming offers many advantages, among which, especially, is an excellent quality of embossing and deformations that must be included on the visible surface, as well as a precise placement of these latter features.
Nevertheless, when the design must be changed, even locally, and/or if one or more specific deformations is/are planned, for example for different versions of the same part, it is necessary to provision as many mold parts (at least for the visible surface) as there are versions of the part, even if some of these versions are not different from each other except in minor or local differences (differences in the size or shape of a reinforcement of a recess for mounting an accessory, management of the presence/absence of locally deformed areas, creation of blanks of areas for mounting or anchoring of projecting equipment or decorative elements, etc.)
Also, the necessity of replacing parts of the mold between two phases of manufacturing of different versions of the same piece drastically reduces the yield and productivity of the tooling and staff, and necessitates the acquisition of specific means to make it possible to change mold parts quickly.
In actual practice, negative thermoforming of a skin or part made of thermoplastic material is generally conducted using, on the one hand, a mold in the shape of a shell, made of nickel and having miniscule holes allowing for the evacuation of air between the sheet to be formed and printed, and the shell. The depression resulting from this vacuum both shapes and engraves the grain of the plate. This shell is produced in a single-block manner by electrodeposition. It is extremely difficult, even impossible, to have an additional part without the jointure between the two parts of the shell being visible in the completed part. If we wish, therefore, optional structures such as housings for a decorative ring, aerators, or startup buttons, at present, as many different shells are required as there are optional versions.
In an attempt to overcome this limitation, the applicant has developed a primary solution in which the complementary part of the mold (in relation to the part of the mold that is in the shape of a shell involved in the negative thermoforming) is equipped with a local hollow shaping part, one that is moveable and connected to a source of vacuum or suction.
When it is activated, consecutive to the general thermoforming of the heated sheet by suction against the first part of the mold, this shaping part uses the sheet, which is in a state that makes its plastic deformation possible, to create a suction cavity that creates the local deformation desired.
This solution is described in the European patent application n° 15305881.3 in the name of the applicant. Although this solution is satisfactory, it has been observed that under certain conditions, as a function in particular of the sequencing of the process, of the type of material the sheet is made of and/or the thickness of this sheet, the skin or preformed part treated by negative thermoforming may, although it may not yet be completely cooled off and the suction on the side of the shell may be shut off, (semi-release of the skin/part between the two parts of the mold), experience shrinkage, with the final dimensions of the resulting part being less than the expected dimensions.